A mother's testosterone levels can determine whether they are more likely to have a tomboy.

Scientists have found that women with higher than average levels of the male hormone testosterone are more likely to have girls who are interested in Scalectrix and toy trucks than Barbie dolls and make-up.

Researchers from City University, London, studied three-year-olds and found some of the tomboys tended to prefer playing with toy trucks and racing cars and showed an early interest in sport.

They were also more likely to choose boys for their close friends.

Mothers with high testosterone levels had girls who were less stereotypically feminine

Professor Melissa Hines

Testosterone levels

But women with low testosterone in pregnancy tended to have girls who displayed typically "girlie" behaviour - playing with dolls, dressing up and using their mothers' make-up.

But the scientists discovered that environmental factors also played a part in creating tomboys like the Enid Blyton heroine George.

"Girlie" girls tend to play more with dolls

Tomboys tended to have older brothers and parents whose behaviour was highly masculine and influenced them as they grew up.

The study, part of a major "nature versus nuture" investigation, looks into how children are affected by their genes and environment.

The scientists, funded by the Wellcome Trust charity, tested blood samples from several hundred pregnant women.

And then when the children reached the age of three-and-a-half the mothers were quizzed again about their child's behaviour.

Gender roles

Professor Melissa Hines, who helped conduct the study, said: "One of the questionnaires they filled in had to do with gender role behaviour.

"The kind of things were whether their children played with dolls or trucks, whether most of their friends were boys or girls, and whether they liked sport.

"What we found was that mothers with high testosterone levels had girls who were less stereotypically feminine.

"It wasn't a huge effect, but it remained even when we controlled for every other factor.

"Girls who were very, very feminine tended to have mothers who were low in testosterone."

She said that the mothers' hormone levels appeared to have no effect on boys, possibly because their hormone levels were already high and because they were under greater social pressure to behave in certain ways.

The research is part of the Avon Longitudinal Study of Parents and Children (ALSPAC).